Land-Mobile Telecommunication systems (e.g., Cellular Radio Telephone), frequently employ base-stations capable of supporting multiple Radio Channel Units (RCUs). Each RCU contains the necessary receiver hardware to support one RF carrier. In order to facilitate a practical base-site antenna system, it is essential that the base-site also contain a Receiver Multicoupler Network (RMN) which permits multiple RCUs to share the same receive antenna or set of antennas. (e.g., Cellular base-sites may have in excess of 100 RCUs).
In order to permit a system operator flexibility in the size and complexity of a particular base-site, it is typically a requirement of the RCU design to function properly in several different RMN configurations. Differences may include the number of available ports, the gain, noise figure, third order intercept point of the network, etc. Additional complexity to the RMN can be introduced if the network is designed to permit switching of multiple antenna inputs to each of the RCUs. (This feature is frequently encountered in Cellular base-sites to facilitate system performance enhancements, such as specific frequency re-use patterns, diversity, etc.)
For any receiver, the sensitivity performance (noise figure) and the intermodulation performance (3rd order intercept point) present opposing requirements on the design of the receiver. High values of gain, particularly in the front-end of the receiver, tend to be beneficial to sensitivity but detrimental to intermodulation (IM) performance. Conversely, moderate to low values of front-end gain produce improved IM performance at the expense of sensitivity.
Simultaneous compliance with both the sensitivity and IM specifications for a given receiver is always an important design consideration, and is often a difficult challenge. Obtaining optimum performance for both parameters in a base-station RCU has several difficulties not had by a mobile design. The complication results from the requirement for the RCU to operate in several different RMN configuration, each of which supports multiple RCUs, and multiple antennas.
The affect of this requirement on sensitivity and IM performance is twofold. First, the high losses associated with the multiple signal splitting stages of the RMN requires corresponding high gain to take over the total noise figure. The composite gain (required to obtain the specified noise figure) of the amplifier stages and the splitting stages is higher than would be required if no splitters were needed. This increased gain is detrimental to IM performance.
The second complication results from the inevitable gain variations of the various RMNs. The requirement for the RCU to operate with multiple receiver front-end configurations brings with it an unavoidable variation in front-end gains. This effect is further extended by the fact that each RMN configuration consists of multiple stages, each with its own gain tolerance as a function of frequency, temperature, and part-to-part variations. The net affect of the front-end gain variation can be simplified as follows: in order to guarantee that the sensitivity requirement is met at the low end of the gain distribution, the RCU must have excess IM margin to deal with the unavoidable IM degradation at the high end of the gain distribution.
Thus a need exists for a receiver/base-station which optimally compromises between receiver sensitivity and intermodulation performance while maintaining specified receiver performance.